Innate immune memory in plants

•The plant innate immune system comprises local and systemic immune responses.•Localized infection, microbial root colonization, or physical injury frequently prime unharmed leaves for enhanced defense and immunity to reinfection.•Plant innate immune memory seems to be inheritable by employing epigenetic mechanisms.•The plant innate immune system builds immunological memory resembling trained immunity in mammals.•There seems to be conservation, or convergence, in the evolution of the innate immune memory in plants and mammals.

The plant innate immune system comprises local and systemic immune responses. Systemic plant immunity develops after foliar infection by microbial pathogens, upon root colonization by certain microbes, or in response to physical injury. The systemic plant immune response to localized foliar infection is associated with elevated levels of pattern-recognition receptors, accumulation of dormant signaling enzymes, and alterations in chromatin state. Together, these systemic responses provide a memory to the initial infection by priming the remote leaves for enhanced defense and immunity to reinfection. The plant innate immune system thus builds immunological memory by utilizing mechanisms and components that are similar to those employed in the trained innate immune response of jawed vertebrates. Therefore, there seems to be conservation, or convergence, in the evolution of innate immune memory in plants and vertebrates.

Graphical abstractSystemic acquired resistance (SAR) and induced systemic resistance (ISR) are the best examined types of systemic plant immunity. ISR is induced upon colonization of the root by certain bacteria and fungi that often also promote growth and yield. Establishment of ISR depends on sensitivity to the plant hormones jasmonic acid (JA) and ethylene (ET). SAR develops upon localized pathogen attack and requires salicylic acid (SA) and pipecolic acid (Pip). The two signals prime the unharmed systemic leaves for enhanced defense, either directly or indirectly, via methyl jasmonic acid (MeJA), dehydroabietinal (DA), glycerol-3-phosphate (G3P) or a G3P derivative, (methyl) salicylic acid ((Me)SA), or azelaic acid (AzA). Defense priming is associated with enhanced levels of pattern recognition receptors (not shown), deposition of dormant signaling enzymes (e.g. mitogen-activated protein kinases (MPK), NONEXPRESSER OF PR1 (NPR1), and WRKY transcription factors), and modifications to chromatin (e.g. de/methylation of DNA and histones). Together, these events provide long-lasting memory to the initial infection resulting in boosted defense and immunity to reinfection.Download full-size image